The present invention relates to a remote sensing apparatus and method having a focal plane occupied by one end of a group of optical fibers, and a plurality of detectors coupled to the other end of the optical fibers such that each detector is advantageously coupled to a single optical fiber in order to accurately measure the intensity of light from a single location in the focal plane.
Remote sensing devices detect and measure energy reflected and/or emanating from remote objects in order to determine physical properties of the objects and, in some instances, to identify the materials from which the remote objects are formed. For example, remote sensing devices operating in the visible and infrared regions of the electromagnetic spectrum are used extensively to measure the characteristics of the Earth's land surface, ocean surface, and atmosphere as a function of position, as well as the characteristics of other bodies in the Solar System. These remote sensing devices use an optical system comprised of one or more optical elements, such as mirrors or lenses, to collect and focus electromagnetic energy from the remote object, such that an image of at least a portion of the remote object is formed in a focal plane of the optical system. This focal plane may in fact be a flat plane, or it may be a curved surface, such as a segment of a spherical or toroidal surface.
A physical assembly located in the focal plane, hereinafter referred to as the focal plane assembly, contains a means of measuring the electromagnetic energy at many points in the focal plane. The area in the focal plane over which these points are distributed is referred to as the extent of the focal plane, and the corresponding area of the remote object that is measured at any one time is referred to as the field of view of the remote sensing device.
The electromagnetic energy measured at each point is integrated over one or more ranges of wavelength. For example, in a panchromatic sensor, energy is measured over a single broad range of wavelengths, as in a black-and-white television camera. In a multispectral sensor, energy is detected in a small number of discrete wavelength bands, typically 4 to 10, with the center wavelength and width of each band selected to detect one or more specific characteristics of the target. A typical multispectral sensor is the NASA Advanced Land Imager, which has 9 bands in the visible, near infrared, and short-wave infrared spectral ranges. In a hyperspectral sensor, energy is detected in a large number of narrow bands, typically 32 to 256. In most hyperspectral sensors, bands are of uniform width and uniformly distributed over a range of wavelengths. A typical hyperspectral sensor is the NASA AVIRIS airborne sensor, which has 224 bands, each 10 nanometers wide, between 0.4 to 2.5 microns.
Certain remote sensing devices, especially those located in satellites, spacecraft, aircraft and other vehicles, employ a type of sensor which is referred to as a “pushbroom” sensor. In this type of sensor the field of view is much wider in one direction, referred to as the cross-track direction, than in the perpendicular direction, referred to as the in-track direction. The field of view is scanned over the remote object(s) by the in-track forward motion of the vehicle.
Another type of sensor is a “line scan” sensor in which the remote sensing device is stationary, but the field of view is moved in the in-track direction by rotating the device or a component of it, such as a mirror. Alternatively, the device and field of view may be fixed and objects may move through the field of view.
In existing realizations of these types of sensors, the energy in the focal plane is measured by an array of photosensitive detectors, such as charge-coupled device (CCD) elements or photodiodes, located at the focal plane and arranged in one or more rows. Each detector generates electrical signals at regular intervals, and each signal is proportional to the energy it has received in the latest interval. By receiving the signals from one or more rows of detectors over a period of time as the field of view is scanned, a processing element can assemble a two-dimensional image of at least a portion of the remote object(s). Such an image is comprised of picture elements, i.e., pixels, where each pixel corresponds to a single measurement from an individual detector.
For some multispectral and hyperspectral remote sensors, there may be 20,000 or more detectors in each row of detectors. A group of detectors, such as one or more rows of approximately 2000 detectors, may be located on a single physical substrate, i.e., a chip, and the chips may be aligned in rows in the focal plane. Each row of detectors may be configured such that it detects only a particular spectral band, i.e., range of wavelengths, of the radiated and/or reflected energy that is received by the detectors. As such, each row of detectors may be located behind a filter of some type that filters out all of the energy except the desired spectral band. For a multispectral sensor, multiple rows of detectors are required. In typical applications, individual detectors have linear dimensions in the range of 5 to 50 micrometers, but additional space must be allowed between rows of detectors for a variety of technical reasons, including but not limited to provisions for electronic circuitry associated with each detector, limits on the minimum width with which spectral band filters can be constructed, and the need to use different substrate materials for different spectral bands. As such, the detectors for each spectral band take up to 1 millimeter of space in the focal plane in the in-track direction. In addition, some sensors require two sets of detector chips, offset in the in-track direction, so that chips can overlap in the cross-track direction to prevent gaps in the image. In one typical multispectral sensor, each chip takes up 10 millimeters of space in the focal plane in the in-track direction (i.e., each chip has an in-track extent of 10 millimeters), and the entire array of detector chips, takes up 20 millimeters of space in the focal plane as a result of their arrangement in two rows.
The in-track extent of the detector array causes the field of view for some detectors to be considerably offset in the in-track direction from the field of view of other detectors. The optical system of the multispectral sensor referred to above has a focal length of approximately 1 meter, so that a 20 millimeter focal plane extent produces approximately 20 milliradians difference in in-track angle of view, i.e., between the direction from which the first detector on the first chip receives energy and the direction from which the last detector on the last chip receives energy at a given time.
The in-track variation in angle of view results in a distance between positions in the scene or target observed by different rows of detectors, and the distance varies depending on the distance from the sensor to the scene or target. The distance from the sensor to the scene may vary in an unknown fashion due to changes in the sensor position (e.g., satellite orbit variations or aircraft altitude changes) or due to variations in surface elevation in the scene. The processing element is required to correct for the difference in scene position observed by different rows of detectors, and this correction becomes more difficult and less accurate as the in-track extent of the detector array increases. For example, for the sensor described above, a 100 meter error in determining the elevation of the scene results in up to a 2-meter error in determining the relative position of pixels from different rows of detectors.
The in-track extent of the detector array also results in a time interval between when the first row of detectors observes a given point on the ground and when the last row of detectors observes the same point. For the sensor described in the above example, which is carried by a satellite at an altitude of approximately 700 kilometers, the maximum delay is approximately 2 seconds. During this time, the satellite or aircraft carrying the sensor can change orientation, i.e., pitch, yaw, or roll, resulting in errors in determining the relative position of pixels from different rows of detectors.
The in-track extent of the detector array also results in a variation in the angle from which a given point in the scene or target is viewed by different spectral bands, which can affect the relative amount of radiation received by the sensor in the different bands.
Furthermore, the in-track extent of the detector array also requires that the optical system provide a high-quality image over this in-track extent, which may be difficult to do while maintaining acceptable optical system performance in other respects, such as aberrations and optical distortions.
To improve the accuracy and reduce the complexity of the corrections that the processing element must perform, it would be desirable to reduce the amount of space utilized by the detectors in the focal plane. As such, various components, such as dichroic (i.e., wavelength-selective) mirrors, may be used to dissect the radiated and/or reflected energy, which is typically visible and/or near and short-wave infrared radiation, into its spectral bands. The spectrally separated radiation then must be directed to the appropriate detectors, such as with lenses or by precise detector positioning. It is difficult, however, to maintain the alignment of mirrors to the detectors because temperature changes may cause the mirrors to slightly change position. In addition, the mirrors require a relatively significant amount of space in front of the focal plane, such that mirrors typically may only be used to separate 2 or 3 spectral bands.
In other embodiments, a wavelength dispersive element, such as a prism, may be used to disperse the energy received via the optical system that is delivered to the detectors, such that the rows of detectors, although physically separated, are optically co-aligned and view the same in-track location in the scene or target. These embodiments, however, are difficult to utilize, particularly when the desired spectral bands are close together or overlap, because the rows of detectors also overlap. Additionally, because wavelength and physical position in the focal plane are directly related in these embodiments, certain design options are precluded. For example, it is not possible to provide two or more rows of detectors that are sensitive to the same spectral band, which is commonly done to improve sensitivity or provide redundancy in case of detector failure. In addition, it is difficult to utilize a dispersive element when the detector array is too wide in the cross-track direction to use a single detector chip, because it is difficult or impossible to align multiple dispersive elements and multiple chips with sufficient accuracy to maintain a uniform spectral response.
Another way to attempt to mitigate the effects of the multiple rows of detectors that create a relatively large focal plane, and require complex corrections, as described above, is to utilize an image conduit. For instance, an image conduit may be made of a group of optical fibers that are fused together and transmit an image from one location to another. As such, the group of fibers may receive the energy at the focal plane, then transmit that energy to a group of detectors. When an image conduit is used, the size of the focal plane may be somewhat reduced, for example by eliminating the additional in-track spacing needed to overlap detector chips in the cross-track direction, or by providing some optical magnification in the in-track direction from the focal plane to the detectors. As such, the corrections that must be made to the readings are decreased, but not eliminated.
The drawback to this configuration, however, is that random subgroups of the optical fibers illuminate each detector, which makes it impossible to control how much light is transmitted to each detector. For large detectors (e.g., 1 mm or larger), the number of image conduit fibers coupled to a detector is large (>10) and detector-to-detector variations can be determined and compensated for. For large arrays of small detectors, however, the number of image conduit fibers coupled to a detector is small, and many fibers couple to two or more detectors. Therefore, the effective sensitivity of each detector is different, and uniform images are difficult to create. In addition, the spatial response of each detector (i.e., the response to a point source of radiation in the scene as a function of the point source position) is complex, and differs from detector to detector. As such, the electrical signals provided by the detectors and read by the processing element, which are proportional to the energy received by the respective detector, may not be consistent, i.e., one detector may be more sensitive to a particular scene or target configuration than another. Therefore, because there is no way to know whether the electrical signals that are read by the processing element are consistent, an incorrect or misleading image of the scene or target may be created.
In other applications (i.e., astronomical spectroscopy), a single fiber has been utilized to transmit energy received at one end to a single detector connected to the other end. Although it is possible to know how much light is transmitted to the detector in the configuration in which one fiber transmits energy to one detector, the drawback is that each assembly of detector and fiber is physically separate, and only a small number of detector/fiber pairs (i.e., 100 or less) may be practically utilized in a sensor, which prevents this configuration from being used in pushbroom imaging applications. As such, there is a need in the industry for a remote sensing apparatus in which a plurality of fibers may transmit energy from a focal plane to a plurality of detectors in such a way that the amount of energy received by each detector is known or may be determined, and therefore, an accurate image of the desired area may be created.